You know how prototype crossing flashers really flash: left - right - left - right. Left and right lamps on for equal periods of time. You probably also know that many model crossing flashers don't work that way -- both sides blink on and off together. AARRGH!

If you'd like your flashers to work prototypically -- or if you've been waiting to install crossing flashers until you found some that worked the right way -- then here's the circuit for you! This simple little jewel will give you real-world flashing operation (adjustable to your tastes, of course). The circuit is shown using LEDs (in whatever size fits your scale), but you could also use sub-miniature incandescents; see "Construction Hints & Tips" below for details on this modification.

As always, all parts are readily available at your local Radio Shack, or from Jameco, DigiKey, etc. Almost nothing is terribly critical, so substitutions from the junk box are allowed; just keep an eye on those non-polarized electrolytics -- they're a must. If you can't find 'em, you can create your own by using two standard electrolytics of twice the value; connect thier plus/positive (+) terminals together (and to nothing else); the minus/negative (-) terminals then become the effective leads of the "non-polarized" cap. Remember, it takes two standard electrolytics to produce one non-polarized beast.

CONSTRUCTION HINTS & TIPS

1. Capacitors C2 & C3 are "non-polarized" electrolytics, often used in speaker crossovers.
2. Resistors R2 & R3 should be of equal value, but may be adjusted for faster or slower operation (ie, larger values will produce a slower flash rate; small values give faster flash rate); likewise, capacitors C2 & C3 should be of equal value, and can be increased for slower flashing, decreased for faster.
3. If you are absolutely certain as to the polarity of your power supply, diode D1 may be omitted; it serves only as a safeguard against momentary reverse connections (which my be fatal to the circuit).
4. Speaking of POWER SUPPLIES, several supplies that would handily power this circuit may be found on the "Power Supplies" page (under "Other Circuits").
5. Most "Block Occupancy Detectors" have open-collector transistor outputs which may be connected directly to the control pin (as long as the Occupancy Detector shares a common ground with the Flasher; "ground" in this case is the negative (-) side of the power supply). If the OD has an internal "pull-up" resistor on the output, the Flasher and OD must share the same power supply (and you can probably omit resistor R5). If you're unclear on this one, get some help first.
6. Resistors R1 & R4 may be reduced somewhat to give greater LED brightness; check the manufacturer's spec for maximum forward current (Imax), and calculate the minimum value of R1 & R4 as follows: Rmin (in Kohms) = 9 / [Imax (in milliamps)]; milliamps are usually abbreviated as "ma".
7. If you can't abide LEDs, and insist on using miniature or sub-miniature incandescent lamps, a few circuit changes may be needed. If you choose a lamp such as Miniatronics' 18-001 (1.5V, 15ma, 1.2mm dia), you may simply substitute it directly for the LED -- no other changes needed. If you choose any other 1.5 volt lamp, you'll need to reduce R1 & R4 according to the formula given in Tip #6, above; use the lamp's specified current (ie, 30 or 40ma) for Imax. If you opt for a 12V, 30ma bulb (eg, Miniatronics 18-712), you should experiment a bit -- simply omit R1 & R4, and connect the bulb as the LED. Finally, if you go with a 12V, 50ma bulb, methinks you're going to want more collector current that the circuit is set up for; to achieve this, I'd substitute a TIP120 NPS Darlington transistor (RS #276-2068) for Q1 & Q2, insert the bulb in place of the LED, and make R1 & R4 something like 22 ohms (just to limit the "inrush current"). If you have something else in mind, contact me and we'll go from there.
8. Flasher may be operated from a 5 volt supply by changing R1 and R4 to 150 ohms (RS 271-1109) and omitting D1 (ensure correct supply polarity -- see Note 3).

COMBINE THE "SIMPLE OPTICAL OCCUPANCY DECTECTOR" (SOOD)
WITH THIS FLASHER TO PROTECT A CROSSING
CIRCUIT w/ LED FLASHER
CIRCUIT w/ INCANDESCENT FLASHER
ALTERNATE CIRCUIT w/ INCANDESCENT FLASHER

It's probably worth mentioning what may be obvious: you can use this Flasher in more ways than just crossings. It can be tweaked to work almost anywhere you want TWO flashing lights. If you're in need of only ONE flashing light, just omit one LED; if you need one higher-current flasher, consider the ROTATING BEACON circuit, also accessed thru the "Other Circuits" page.

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Updated 7-10-02